Even with the DI box, your sound card is going to want a port for a BIG jack... y'know, the ones guitar leads have - if you don't want all the high end of your guitar signal wiped out, so you might need an upgrade in that area.

As for DIs themselves, different instruments require different pieces of hardware - recording guitars with a DI designed for bass ain't gonna cut it, in other words.

Line 6 do a nice thing called the Toneport which connects to the PC via USB. You can record everything through that so long as your computer is fast enough. And there are some very nice sounding presets too. Check it out.